The SB-800 is a great flash, but onehasto take into consieration 3 tings:
- budget
- requirements
- future needs
The SB-800 can be triggered remotely by the onboard flash of the D50 for great off caera flash, it is VERY powerful for a strobe and gives you the most options, however if you are not an avid flash user, it is expensive.
The SB-600 is cheaper, less powerful and I think can be triggered optically as well for WAY better results with off camera flash, but you cannot use iTTL or CLS. It would have to be configured manually.
Depending on what you want to do, either the SB-800 or SB-600 will do the trick, however the SB-800 is above your budget.
Looks like for you, the SB-600 is the best choice.